Why the Economizer Does Not Steam in Packaged Boilers
Unlike HRSGs, packaged boilers, fortunately, do not have to deal with the issue of steaming. The reason is illustrated in Fig. 3.25, which shows the temperature
FIgure 3.25 Economizer temperature pick-up in boiler versus HRSG.
Profiles of the economizer of the boiler whose performance is given above and an HRSG.
Because of the small ratio of gas to water flow in packaged boilers, the temperature drop of the flue gas has to be large for a given water temperature increase. If the water temperature increases by, say, 145°F, the gas temperature drop is given by
1.23 x 0.286 x (Tj — T2) = 145 or
T — T2 = 412°F
Whereas in an unfired HRSG, the gas temperature drop of only 105°F accomplishes a water temperature increase of 248°F! Thus it is easy for the water to reach saturation temperature in HRSGs. Thus in spite of the fact that the gas entering temperature is quite large in packaged boilers (due to the high furnace exit gas temperature), the water temperature does not increase significantly
If the water temperature approach is large at 100% load, it will be even larger at partial loads, because the gas temperature entering the economizer decreases.